As has been long known, vanadium-containing fuel oil ashes can be treated with mineral acid to dissolve the vanadium. Recovery is improved by adding a reducing agent to the leach solution prior to filtering to remove the ash residue from the acidic leach liquor. But this procedure is useful to advantage only if the vanadium recovery need not be so high that other metal values in the ash such as nickel and magnesium interfere with vanadium separation, making more complex and expensive vanadium separation steps necessary.
Another procedure for recovering vanadium from such ash containing 10 to 80% carbon involves selectively dissolving the vanadium in a caustic soda solution. An oxidizing agent is used in sufficient quantities to oxidize the vanadium as reduced vanadium is difficult to dissolve under alkaline conditions. The nickel and magnesium are left behind in the ash residue as vanadium is removed from the solution by solvent extraction, ion exchange or precipitation. But when the ash is that of natural bitumen and contains 10% or more of magnesium as the sulfate, for example, reagent consumption must be high in order to obtain soluble vanadium recoveries as high as 80-90%. Additionally, leaching at high base concentration is required for efficient reaction rate and further significantly increases the cost of the alkali leach process.